1. Field of the Invention
The present invention relates generally to a high frequency oscillator for generating frequencies in millimeter-wave and micro-wave bands, and more particularly, to a high frequency oscillator which uses both a balanced transmission line and an unbalanced transmission line to combine outputs of a first and a second oscillator which oscillate at the same oscillation frequency.
2. Description of the Related Arts
A high frequency oscillator which generally oscillates at frequencies in a range of 1 GHz to 100 GHz is useful, for example, as an oscillation source for a high frequency circuit network associated with optical fibers, or as an oscillation source for measuring devices. As disclosed, for example, in Japanese Patent Laid-open Application No. 4-175001 (JP-A-4-175001), one type of the high frequency oscillator mentioned above is a so-called push-push oscillator which uses two oscillators, each of which oscillates a fundamental wave, and combines outputs of these oscillators to generate a component at a frequency twice as high as that of the fundamental wave. The oscillator which generates a frequency component twice as high as the fundamental wave is hereinafter called the xe2x80x9csecond harmonics oscillatorxe2x80x9d.
FIG. 1 illustrates the configuration of an example of conventional second harmonics oscillator. The second harmonics oscillator comprises resonator 1, first oscillation unit 2a, second oscillation unit 2b, and combiner 3.
Resonator 1 uses, for example, a dielectric resonator, or LC elements or a micro-strip line built in an IC (integrated Circuit). Oscillator units 2a, 2b each include an amplifier for oscillation, and a feedback system, and share resonator 1 to form a first and a second oscillator, each of which has the same oscillation frequency f0 of a fundamental wave. The respective oscillators are designed such that the fundamental waves delivered therefrom have a phase difference of 180 degrees, i.e., opposite phases to each other. Combiner 3, which may be an in-phase combiner comprising, for example, a differential amplifier, combines outputs of the respective oscillators to supply combined output fout to the outside.
FIGS. 2A to 2C show output waveforms of the output of first oscillation unit 2a, the output of second oscillation unit 2b, and combined output fout, respectively. In the circuit illustrated in FIG. 1, fundamental waves f0 delivered from first and second oscillation units 2a, 2b are out of phase by 180 degrees as shown in FIGS. 2A and 2B, so that the fundamental wave components in combined output fout are canceled by each other and removed, as shown in FIG. 2C. On the other hand, first and second oscillation units 2a deliver second harmonic waves (2f0) each having the frequency twice as high as that of fundamental wave f0 are in phase, so that they are combined by combiner 3 which delivers combined output fout. Consequently, the circuit illustrated in FIG. 1 cancels and therefore eliminates the fundamental wave and odd-numbered order harmonics of oscillation components from respective oscillation units 2a, 2b. Even-numbered order harmonics from respective oscillation units 2a, 2b are combined so that the respective components have levels twice as high. However, even-numbered order harmonics of fourth or higher have significantly low levels as compared with the second harmonic component, so that second harmonic wave 2f0 presents a maximum level in combined output fout delivered from the circuit.
The second harmonics oscillator illustrated in FIG. 1, however, has a problem in a reduction in size, which is difficult to solve because combiner 3 is required for combining the outputs of first and second oscillation units 2a, 2b. Further, since oscillation frequency components, i.e., the fundamental wave components, from first and second oscillation units 2a, 2b, including combiner 3, must be maintained in opposite phase, i.e., with a phase difference of 180 degrees with each other, difficulties are encountered in electrical circuit designing.
It is an object of the present invention to provide a high frequency oscillator which promotes a reduction in size and facilitates the designing for opposite phase oscillation.
The object of the present invention is achieved by a high frequency oscillator for combining outputs of two oscillators to generate an oscillation output. The oscillator has a substrate, a slot line formed on a first main plane of the substrate and having both longitudinal ends, the both longitudinal ends being electrically short-circuited, a first and a second amplifier for oscillation, each disposed on one or the other side of the slot line, and having outputs of the same oscillation frequency, and an unbalanced transmission line for connecting input terminals of the first and second amplifiers to each other and for connecting output terminals of the first and second amplifiers to each other. The unbalanced transmission line traverses the slot line and forms a closed loop including the first and second amplifiers.
In the present invention, high frequency waves from the output terminals of the first and second amplifiers through the unbalanced transmission line are converted in its propagation mode by the slot line from an unbalanced mode to a balanced mode, and propagate toward one end and the other end of the slot line. In this case, the high frequency waves converted to the balanced mode propagated in opposite phases to each other in both sides of the slot line. The high frequency waves received by the unbalanced transmission line which connects the input terminals of the amplifiers to each other are again converted to the unbalanced mode. Then, the high frequency waves are branched into two from a midpoint of the slot line in the width direction in opposite phases to each other, and fed back to the input terminals of the first and second amplifiers, respectively. Consequently, the first and second amplifiers oscillate in opposite phases to each other, and a first and a second oscillation closed loop in opposite phases to each other are formed. Therefore, assuming that the fundamental wave of oscillation has a frequency f0 in each oscillation closed loop, a second harmonic component (2f0) can be readily generated on the principle of push-push oscillator.
According to the present invention, the high frequency oscillator can be realized in plane circuit configuration without the need for a combiner and the like, thereby contributing to a reduction in size and a simple design for opposite phase oscillation.